He said: “Why the hysteria around legal highs, particularly drugs such as nitrous oxide [which] in its 200 year history hasn’t killed anyone? One reason for this seems to relate to the rise of the ‘head shop’ in many town centres. These are viewed like sex-shops as lowering the tone of localities and increasing public disorder (though never to the extent seen with premises selling alcohol).

“Another aspect is the opportunistic vilification of youth culture by the right-wing media who have labelled nitrous oxide as ‘hippy crack’, even though everyone knows it is very much less harmful than crack cocaine and no self-respecting hippy would be seen using it!”

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He added: “By trying to ban safe legal highs it moves the law from one that reduces harm to one that tries to control moral behavior. I would argue this is the worst assault on personal freedom since the 1559 Supremacy Act decreed that the practice of Catholic beliefs was illegal.”

Professor Nutt was sacked in 2009 after repeatedly criticising the then-Labour Government's approach to drug classification, including saying riding a horse was more dangerous than taking ecstasy.

Under the Bill, suppliers or producers of legal highs could face up to seven years in prison.

Irish police have to scientifically prove the substances produce a psychoactive effect, and there have only been four successful prosecutions in the past five years.

Critics of the Bill argue banning legal highs will force the industry “underground”, where people will be exposed to harder, more addictive drugs.

The amendment to the Bill in the Lords today, tabled by former Metropolitan Police Deputy Assistant Commissioner Lord Paddick, would mean the Home Secretary would have to commission "an independent evidence-based review of the implementation of the Misuse of Drugs Act 1971", with a report published within a year of the Psychoactive substances Bill receiving Royal Assent.

Speaking in the Lords this afternoon, Lord Paddick said: “The Liberal Democrats are as concerned about the harm caused by the misuse of drugs and the misuse of new psychoactive drugs in particular as anyone in this House.”

He added: “Liberal Democrats want what parents and families want. Parents want their children to avoid taking drugs. The evidence suggests that education, rather than criminalisation, is more likely to achieve that end.”

Former Tory Home Affairs minister Lord Blencathra spoke out in favour of the legislation, and pointed to the findings of a report produced by Lord Hannay on the EU’s drug strategy in 2012.

He said: “Enforcement has worked extremely well for all the main hard drugs we have had in this country. Drug use of heroin, crack cocaine, and those drugs has dropped dramatically, but where we are in the lead unfortunately is in the new psychoactive substances drugs. It would seem from the evidence we took in the committee that children don’t want to take the same old stuff their hippy fathers did. So if it was good enough for dad, the kids today want something different.

“We see that in a whole range of things, from children going off Facebook because their parents have joined. The fads on drugs seem to have the same trends.”